Field of the Invention
The invention relates to a process for operating a PEM fuel cell installation.
It is known that, during the electrolysis of water, water molecules are decomposed by electric current into hydrogen (H2) and oxygen (O2). In a fuel cell, that process takes place in reverse. Electric current is produced with high efficiency through an electrochemical combination of hydrogen (H2) and oxygen (O2) to form water. If pure hydrogen (H2) is used as combustion gas, it takes place without the emission of pollutants and carbon dioxide (CO2). Even with a technical combustion gas, for example natural gas or coal gas, and with air (which may additionally be enriched with oxygen (O2)) instead of pure oxygen (O2), a fuel cell produces considerably less pollutants and less carbon dioxide (CO2) than other forms of energy production which operate by using fossil energy sources. The technical implementation of the fuel cell principle has given rise to a wide variety of solutions, and more precisely with different electrolytes and with operating temperatures of between 80xc2x0 C. and 1000xc2x0 C.
The fuel cells are classified as low, medium and high temperature fuel cells according to their operating temperature, and they in turn differ for a variety of technical embodiments.
Besides the aforementioned fundamental advantages, a fuel cell with a plastic solid electrolyte (polymer electrolyte membrane or PEM) offers further positive properties such as low operating temperature ( less than 80xc2x0 C.), favorable response to overloading, little voltage degradation and long life span, favorable response to loading cycles and temperature cycles, and the absence of a corrosive liquid electrolyte. It is furthermore suitable for operation with ambient air instead of oxygen (O2). Together, those properties make an air-operated PEM fuel cell a virtually idea producer of energy, for example for electrically powering a motor vehicle without producing exhaust gases.
A PEM fuel cell block (the fuel cell block is also referred to as a xe2x80x9cstackxe2x80x9d in the specialist literature) is generally composed of a large number of PEM fuel cells which have a planar structure and are stacked together. Since the PEM fuel cell block is not operable on its own, the PEM fuel cell block, an operating part and associated module electronics are generally combined to form a PEM fuel cell module. The devices for supplying working media, for example hydrogen (H2) and oxygen (O2) or air, for discharging the water which is produced, for dissipating heat losses, for wetting the working media and for separating gas impurities, are combined in the operating part.
If the anode and the cathode of the PEM fuel cell are supplied with their working media, then a cell voltage is created from the sum of the anode and cathode potentials, which has a specific characteristic depending on the load current. In order to ensure an orderly build-up of all of the cell voltages in the PEM fuel cell block when switching on, the supplying of the anodes and cathodes with their working media must be established through a defined switch-on phase, so as to make it possible to obtain a delay-free changeover from the switch-on phase to a phase for producing electrical energy, in other words, a load phase. When switching off, the supplying of the PEM fuel cell block with the working media is suspended, and the residual capacity in the gas spaces of the anodes and cathodes is taken down by loading, so long as the voltage of the PEM fuel cell block remains greater than 0 V.
In one process for switching off a PEM fuel cell module, which is known from the prior art, a hydrogen inlet valve is closed in a first step and an oxygen inlet valve is closed in a second step, according to the hydrogen partial pressure at an anode part. That process leads to an increase in the internal resistance of the PEM fuel cells and poisoning of the electrolyte membrane, which is equivalent to a power loss and causes premature failure of the PEM fuel cell module.
It is accordingly an object of the invention to provide a process for operating a PEM fuel cell installation, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known processes of this general type and which avoids such premature fuel-cell aging as far as possible.
The basis of the invention is that, if the voltage of the PEM fuel cell block is 0 V, this only means that the sum of all of the cell voltages is 0 V. Very different cell voltage profiles may arise during switch-off and in the following off phase, depending on how the individual PEM fuel cells in the PEM fuel cell block are connected in terms of the flow of working media, and depending on what load is set in the switch-off phase.
These cell voltage profiles firstly indicate non-uniform supplying of the anodes and cathodes with working media during the switch-off phase. As a result of subsequent diffusion of the working media through the electrolyte membranes, only that working medium which is present in excess remains, since both the anode and the cathode work unselectively.
The gas fillings at the anodes and at the cathodes are of approximately the same size for structural reasons. If, for example, hydrogen (H2) and oxygen (O2) are used as working media, then twice as much oxygen (O2) as oxygen (O2) is consumed during the electrochemical reaction in the PEM fuel cells. If the supply of hydrogen and the supply of oxygen are then suspended simultaneously, or else the supply of hydrogen is suspended before the supply of oxygen, oxygen (O2) will be present in excess in the PEM fuel cell block.
That residual oxygen (O2) in the PEM fuel cell block sets processes in motion at the components of the PEM fuel cells which cause a reduction in the cell voltage and therefore also in the efficiency. Irreproducibly created oxide layers change the internal cell resistance and therefore the current density distribution. In that case, corrosive processes may also occur, which poison the electrolyte membrane and thereby shorten the life span of the PEM fuel cell block. Both the increase in the internal cell resistance and the corrosion on the components cause a reduction in the cell voltage.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for operating a PEM fuel cell installation, which comprises providing at least one or each PEM fuel cell module with a device for adjusting a supply of hydrogen and a supply of oxygen; and suspending the supply of working media (hydrogen H2 and oxygen O2 in this case) by suspending the supply of oxygen in a first step and suspending the supply of hydrogen in a second step.
By virtue of this process, the fuel cell is preserved in a condition which guarantees reliable operation of the PEM fuel cell installation over relatively long periods of service. During suspension, potentials at the electrodes of the PEM fuel cells, which could cause corrosive processes at the components of the PEM fuel cells, are avoided. A voltage loss due to an increased internal resistance of the PEM fuel cell and a reduction in the life span of the PEM fuel cell module are thereby avoided. Sufficient stability of the cell voltages is furthermore achieved.
In accordance with another mode of the invention, in the second step, the supply of hydrogen is suspended when a predetermined oxygen partial pressure at a cathode part of the PEM fuel cell module is reached.
In accordance with a further mode of the invention, a predetermined value of about 0.5 bar for the oxygen O2 partial pressure has proved suitable.
In accordance with an added mode of the invention, the oxygen (O2) may be supplied in the form of compressed air. When operating with air, an air compressor may be used to take in air from the surroundings and feed it into the PEM fuel cell module. In the first step, it is then merely necessary to switch off this compressor.
In accordance with a concomitant mode of the invention, the oxygen partial pressure is measured indirectly through the static air pressure.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for operating a PEM fuel cell installation, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.